This invention relates to an improvement in sealed lead-acid batteries.
Sealed lead-acid batteries that make use of a phenomenon called "oxygen cycle" (i.e. oxygen gas evolved during charging is absorbed by the negative electrode) are available in two types, a retainer-type and a gel-type. In a retainer type sealed lead-acid battery, a mat separator (glass separator) made of fine glass fibers is inserted between a positive and a negative plate in order to retain the necessary amount of sulfuric acid electrolyte for charging and discharging the battery and for keeping the electrodes apart. The retainer-type battery has many advantages such as maintenance-free operation, no electrolyte leakage and attitude independence. Consequently, this battery has recently been used in increasing volume as a power source for portable equipment, cordless convenience devices and computer backups. The glass separator used in retainer-type sealed batteries is a mat of very fine glass fibers of a diameter of approximately 1 micron or less, which are made by a special technique. Therefore, the separator is considerably more expensive than separators in common use with lead-acid batteries. Furthermore, in order to attain the desired battery performance, an assembled element must be compressed into a container with great force, which makes the assembly of a battery difficult and unavoidably increases the cost of battery production. Another problem with retainer type sealed lead-acid batteries is that sulfuric acid electrolyte is only retained in the glass separator inserted between positive and negative plates, so less electrolyte can participate in the charge and discharge cycles. Compared to open type common lead-acid batteries, the capacity (in particular the low-rate discharge capacity) of the retainer-type ones is low because sulfuric acid used in lead-acid batteries is one of the active material and the battery capacity is limited by the amount of electrolyte. The positive and negative plates in retainer-type sealed lead-acid batteries are usually made of grids of antimony-free lead alloy having high hydrogen overvoltage. Repeated "deep" charge and discharge cycles of these batteries forms a passivated layer of poor conductivity at the interface between the positive grid and the positive active material, leading to premature loss of the battery capacity. This phenomenon can be prevented by using the positive grid of a lead alloy containing a small amount of antimony. However, the addition of antimony lowers the hydrogen overvoltage and increases the amount of water loss during charging to such an extent that the life of the battery is exhausted prematurely and the batteries dry up, which is fatal to sealed lead-acid batteries. Hence, it has been impossible to use antimony-containing lead alloys in sealed lead-acid batteries that make use of an oxygen cycle. Compared to the retainer-type, gel-type lead-acid batteries are inexpensive but their life performance has been lower than the retainer-type and open type common lead-acid batteries that use a sufficient amount of electrolyte.